1. Field of the Invention
The invention relates in general to displays, and in particular to a fixing apparatus for reducing the undesired noise inside the backlight unit, particularly noise of the diffuser plate or other optical sheets of the backlight unit when the LCD module is shaken.
2. Description of the Prior Art
With the advantages of handy size, light weight, low power consumption and no radiation contamination, liquid crystal displays (“LCD”) have been widely used in recent years. In the commercial market, the LCD applications include the portable products such as personal digital assistants (PDA), regular size products such as monitors of laptop or desktop computers, and large size products such as 30″˜40″ LCD-TVs.
The LCD is assembled by a display unit and a backlight unit housed in a case with a mold frame. The display unit comprises a display panel, a printed circuit board for processing the signals, and a tape carrier package (TCP) for providing mechanical support. The backlight unit disposed under the display unit provides a uniform light to the display unit of the LCD.
Backlight units are generally divided into two types: a side edge backlight and a direct type backlight, according to the disposition of the light source. Generally Speaking, the side edge backlight unit is applied to the small or regular size (about 20″ or less) LCD, and the direct type backlight unit is applied to the large size LCD or TV monitor. The side edge backlight unit, for example, comprises a light source such as the cold cathode fluorescent lamps (CCFL) or the spot light disposed near the sidewall, a light guiding plate, a diffuser plate (for amplifying the light) and the optical sheets for illuminating the light uniformly and increasing the lighting efficiency. The LCD TV requires higher standard at brightness and wide viewing angle; thus, the direct type backlight unit having more lamps disposed below the display panel becomes widely used in the large size LCDs or TV monitors. Also, only the diffuser plate and the optical sheets are required in the direct type backlight unit, it is no need to use the light guiding plate since the light straightly comes from the bottom of the backlight unit. Accordingly, the direct type backlight unit requires fewer components than the side edge backlight unit.
FIG. 1 schematically shows a cross sectional view of a conventional direct type backlight unit. The direct type backlight unit at least comprises a casing 10, a reflective plate 12, the light source 14, a diffuser plate 16 and several diffuser films 18. The light source 14 (e.g. U-shaped Lamp) arranged in the casing 10 emits the light upward, and the light is uniformed by the diffuser plate 16 and the diffuser films 18. The reflective plate (e.g. an aluminum plate) disposed at the bottom surface of the casing 10 is used for minimizing the loss of light and increasing the light efficiency. The backlight unit is further assembled with the display unit (not shown in FIG. 1) to complete a LCD.
When the product having the backlight unit is in use, the diffuser plate 16 and the diffuser films 18 are thermal and moist expanded; thus, it is important to make a space between the diffuser plate 16 (diffuser films 18) and the frame 11 of the casing 10. Otherwise, the diffuser plate 16 (diffuser films 18) is deformed and bended upward to press the display unit after thermal or moisture expansion. This in turn has a bad influence on the display quality of the LCD.
Such thermal expansion (ΔLt) is given by the equation (1):ΔLt=L×ΔT×α1  (1)where L is total length, ΔT is temperature difference and α1 is thermal expansion coefficient.
The moisture expansion (ΔLm) is given by the equation (2):ΔLm=L×α2  (2)where L is total length and α2 is moisture expansion coefficient.
Take a 30″ diffuser plate made by polycarbonate (PC) as an example. The space g1 saved for the 30″ diffuser plate and relating to the length of the diffuser plate can be determined by the following calculation.
It is assumed that the temperature is changed between 25° C. (room temperature) and 70° C. The 30″ diffuser plate is 664.8 mm in length. The thermal expansion coefficient α1 of polycarbonate is about 6˜8×10−5 mm. The moisture expansion coefficient α2 of polycarbonate is about 0.08%. The mechanical tolerance of polycarbonate is about 1.0 mm.
The thermal expansion (ΔLt) and the moisture expansion (ΔLm) are:ΔLt=L×ΔT×α1=664.8×(70−25)×7×10−5=2.09 (mm)ΔLm=L×α2=664.8×0.08%=0.53 (mm)Thus, the space g1 saved for the 30″ diffuser plate and relating to the length of the diffuser plate is:g1=(2.09+0.53+1.0)/2=1.81 (mm)
Similarly, the space g2 saved for the 30″ diffuser plate and relating to the width of the diffuser plate can be determined by using the equations (1) and (2).
According to the description above, the LCD in larger size requires the larger diffuser plate and the spaces (g1 and g2) saved for adapting the expansion of the diffuser plate. However, the diffuser plate 16 and the diffuser films 18 of the conventional backlight unit are stacked instead of fixing on the frame 11. After the display unit is assembled with the frame 11, the diffuser plate 16 and the diffuser films 18 in a stack can be kept within the LCD. If the LCD is moved or shaken, the diffuser plate 16 and the diffuser films 18 will strike the edges of the frame 11 due to the existence of the spaces (g1 and g2), so as to make the abnormal sound (i.e. noise). The larger diffuser plate (which means it is bulky) causes the bigger noise. Also, the larger spaces saved for the larger diffuser plate amplifies the noise defect.
Therefore, there is a need in the art for improved display apparatuses.